Protein LHCII

Figure 23-30 Views of light-harvesting protein LHCII of green plants. (A) Side view indicating the approximate position in the lipid bilayer of the thylakoid membrane. Helices are labeled A-D. (B) Stereoscopic top view from the stromal side of the membrane. The structure, at 0.34 nm resolution, was determined by electron crystallography on highly ordered two-dimensional crystals. MolScript drawings from Kuhlbrandt et al.3W Courtesy of Werner Kiihlbrandt.

The structure of the major trimeric LHCII complex has been recently obtained at 2.72 A (Figure 7.3) (Liu et al., 2004). It was revealed that each 25kDa protein monomer contains three transmembrane and three amphiphilic a-helixes. In addition, each monomer binds 14 chlorophyll (8 Chi a and 6 Chi b) and 4 xanthophyll molecules 1 neoxanthin, 2 luteins, and 1 violaxanthin. The first three xanthophylls are situated close to the integral helixes and are tightly bound to some amino acids by hydrogen bonds to hydroxyl oxygen atoms and van der Waals interactions to chlorophylls, and hydrophobic amino acids such as tryptophan and phenylalanine. 

Neoxanthin and the two lutein molecules have close associations with three transmembrane helixes, A, B, and C, forming three chlorophyll-xanthophyll-protein domains (Figure 7.5). Considering the structure of LHCII complex in terms of domains is useful for understanding how the antenna system works, and the functions of the different xanthophylls. Biochemical evidence suggests that these xanthophylls have a much stronger affinity of binding to LHCII in comparison to violaxanthin... 

A close analysis of the trimers order in the crystal revealed that the exposed part of neoxanthin molecule is completely free from interactions with any protein or pigment components (Pascal et al., 2005). In addition, an examination of the neoxanthin configuration, taken from the structure of LHCII, points toward strong distortion of the d.v-end of the molecule (Figure 7.9). This fact suggests that the twist most likely occurs within the protein interior, implying that some movement in the LHCII monomer must take place during the transition into dissipative state. Apparently, this movement affects not only lutein 1, as previously discussed, but also neoxanthin. 

The v4 region enhancement and structure in the resonance Raman spectra of xanthophylls reviewed in this chapter shows that it can be used for the analysis of carotenoid-protein interactions. Figure 7.8 summarizes the spectra for all four major types of LHCII xanthophylls. Lutein 2 possesses the most intense and well-resolved v4 bands. The spectrum for zeaxanthin is very similar to that of lutein with a slightly more complex structure. This similarity correlates with the structural similarity between these pigments. It is likely that they are both similarly distorted. The richer structure of zeaxanthin spectrum may be explained by the presence of the two flexible P-end rings... 

Mullineaux, C.W., Pascal, A.A., Horton, P. and Holzwarth, A.R. 1992. Excitation energy quenching in aggregates of the LHCII chlorophyll-protein complex A time-resolved fluorescence study. Biochim. Biophys. Acta 1141 23-28. 

The CP26 pigment-protein complex has been described in maize and spinach as having an intermediate chib content between CP29 and LHCII [8, 75,76]. Its pigment complement includes violaxanthin, lutein and neoxanthin as well as chla and chib in a 2.2 ratio [10]. Lower (1.8) and higher (2.7) a/b ratio... 

The above described data have been summarized by different laboratories into structural models. In Fig. 2 the most recent proposals are shown. They all agree on the point that the minor pigment-proteins should be located in a pericentral position between the core complex and the major LHCII. Major differences are encountered in the monomeric (B) or dimeric (A, C, D) organization of the core, the stoichiometry between the core complex and LHC proteins (model in panel D uses the same stoichiometry as the one in panel A) and, finally, the possibility that CP29 and CP24 belong to the same subcomplex (A, C), as previously suggested [9, 10, 63], or are independently connected to the core complex (B, D). 

Horton, R, Ruban, A. V., Rees, D., Rascal, A. A., Noctor, G., and Young, A. J. 1991. Control of the light-harvesting function of chloroplast membranes by aggregation of the LHCII chlorophyll protein complex. Eebs Lett. 292 1-4. 

The unique water-soluble peridinin- Chi a-protein (PCP) complexes are found in many dynoflagellates in addition to intrinsic membrane complexes. [64] It contains Chi a and the unusual carotenoid peridinin in stoichiometric ratio of 1 4. Unlike other families of antennas, the main light-harvesting pigments are carotenoids, not chlorophylls. The structure of the PCP consists of a protein that folds into four domains, each of which embeds four peridinin molecules and a single Chi a. The protein then forms trimers, suggested to be located in the lumen [64] in contact with both LHCI and LHCII [66], allowing efficient EET to occur. 

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